N-hydrocarbon-substituted ureas as anti-exposure cracking agents for sulfur vulcanizable rubbers



Stats N-HYDROCARBON-SUBSTITUTED UREAS AS ANTI-EXPOSURE CRACKING AGENTS FOR SULFUR VULCANIZABLE RUBBERS N Drawing. APPlicationFebruary 9, 1953, Serial N0. 336,014

11 Claims. (Cl. 260-459) This invention relates to new anti-exposure cracking agents for sulfur-vulcanizable rubbers and to rubber vulcanizates containing same.

One of the major problems of the rubber industry is the protection of vulcanizates of sulfur-vulcanizable rubbers against exposure cracking, that is degradation of the vulcaniza-te due to ozone while under static or dynamic stress. The problem is a serious one, particularly with respect to the service life of rubber tires, and is aggravated by the fact that in general chemical antioxidants have little or no value in protecting vulcanizates against deterioration by ozone. Although many and varied substances have been suggested and tried, not one has been found entirely satisfactory. Accordingly rubber technologists have constantly sought improvements.

In accordance with this invention it has been found that compounds having the structure \N-CX.-N/ RI RII where X is sulfur or oxygen and R, R, R" and R are hydrogen or hydrocarbon groups, at least two of them being alicyclic, are etfective anti-exposure cracking agents ior sulfur-vulcanizablerubber vulcanizates.

Of this new family of anti-exposure cracking agents those of the general formula where R is an aromatic hydrocarbon radical are particularly effective and represent a preferred embodiment of this invention. As illustrative of the preferred antiexposure cracking agents of this invention are 1,1-dilcyclohexyl-3-phenylurea 1,1-di(methylcyclohexyl) -3-phenylurea 1, 1-dicyclohexyl3-( l-naphthyl urea 1, l-dicyclohexyl-3-(2-naphthyl)urea l,1-dicyclohexyl3-p-tolylurea 1,1-dicyclohexyl-3-biphenylylurea Other urea compounds exhibiting efiective anti-exposure cracking properties in sulfur-vulcanizable rubber vulcanizates are 1, l-dicyclohexyl-3 -phenylurea 1, l -dicyclohexyl-3-( 1-naphthyl)-2-thiourea 1, l -dicyclohexyl-3- 2-naphthyl) -2-thiourea 1,1,3-tricyclohexyl-3-p-tolylurea 1,1-dicyclohexyl-3-m-tolylurea 1-tert-butyl-3,3 -dicyclohexylurea 1,1-dicyclohexyl-3-(3,5-xylyl) urea 1,1-dicyclohexyl-3-tetrahydronaphthylurea 1,1-dicyclohexyl-3-p-tert-butylphenylurea 1,1-dicyclohexyl-3-p-ethylphenylurea 1,1-dicyclohexyl-3-p-tolyl-2-thiourea 1, l-dicyclohexyl-3-ethylurea 1-amyl-3 ,3-dicyclohexylurea 1,1-dicyclohexyl-3 -hexylurea A tent 2,766,219 li atented Oct. 9, 1956 1,3-dicyclohexyl-1phenylurea l,1-dicyclohexyl-3 -rnethylcyclohexylurea 1-,cyclohexyl-3,Sdieyclohexylurea 1-cyclopentyl-3,3-dicyclohexylurea Stock gppcomwoc dream Smoked sheets rubber parts-'byweight Carbon black ..d Saturated hydrocarbon softener d0. Zinc oxide do.. Stearlc-aeid. Sulfur Anti-exposure cracking agent A.

The ingredients are admixed on a rubber mill in the customary fashion and the respective compounded stocks are cured in a press at 142 C. for '30 minutes. Since evaluation under static conditions is not indicative of the service-obtained with many types of rubber articles which must withstand flexing, the vulcanized compositions are evaluated under dynamic conditions in an atmosphere containing a definiteconcentration of ozone. Samples of the stocks are cured in the form of a belt /2 wide, A" thick and 5 diameter and mounted 1011 l" diameter shafts. The ozone concentration is maintained at 20-30 parts per hundred "millionthroughout the test and the shafts are rotated at R. P. M, In this manner a momentary elongation through a range of 020% is provided at any portion of the test-specimen passing over the shaft. (The apparatus and procedure employed is described in apaper of Creed et *al. entitled An Apparatus for the Evaluation of Ozone Protective Agents for Elastomers Under Dyna-mic'Conditions, given at the Cincinnati, Ohio meeting of the rubber Division of the American Chemical Society May '1, 1952;) The experimental test specimens are compared visually at various intervals noting the extent Of cracking. A stock which is severely cracked has no service life remaining in terms of the useful life of a rubber article and Where the cracking is designated as extremely severe the degradation is well beyond even this point. The results of the tests are set forth below: I

Table 1 Oracking Agent Stock 8Hrs. 24Hrs. 48 Hrs. I 56 Hrs.

none dO None 1,1-Dicyclohexyl-3 phenylurea.

Slight none severe. slight.

The new compounds of this invention are, particularly useful in the control of exposure crackingof the sulfurvulcanizable synthetic rubber-like materials prepared by the polymerization of an aliphaticconjugated diene compound, either alone or with other unsaturated compounds copolymerizable therewith. The synthetic rubbers which may be substantially improved in ozone resistance are polymers of butadiene, isoprene, piperylene, dimethyl butadiene, ethyl butadiene, and the like either alone or with unsaturated mono-olefinic compounds which contain the CH2 C group such as the 'vinyl aromatics, namely styrene, a-methyl styrene, nuclear substituted styrenes, monochlorstyrene, dichlo'rstyrene, divinyl benzene, vinyl naphthalene, vinyl biphenyl, vinyl carbazole, 2-vinyl-5-ethyl pyridine, etc., and such vinyl compounds as acrylonitrile, methacrylonitrile, acrylic acid, methacrylic acid and esters thereof, methyl vinyl ketone, and the like.

As illustrative of the control of exposure cracking of vulcanized sulfur-vulcanizable synthetic rubber-like materials, rubbery butadiene-1,3-styrene copolymer compositions are prepared utilizing the following ingredients:

The ingredients are admixed on a rubber mill in the customary fashion and the respective compounded stocks are cured in a press at 144 C. for 30 minutes. The vulcanizates are then evaluated for cracking resistance as aforedescribed. In Table II the ratio of the cracking resistance of Stock D to that of an untreated control (Stock C) is recorded. This is a very convenient method of summarizing the data. The data were obtained by assigning a numerical rating as a measure of the degree of cracking and plotting these units on the vertical axis against the time of exposure. The numbers employed and the corresponding descriptions were as follows:

1. No cracking 2. Very slight cracking 3. Slight cracking 4. Moderate cracking 5. Severe cracking 6. Very severe cracking A set of stocks answering this description was used as a standard and each stock was compared to this standard so as to assign a comparable numerical rating. The six degrees of cracking were designated on the vertical axis in reverse order, in other words beginning with six nearest the horizontal axis and ending with one at the top. Plotting the numerical ratings against exposure time in this manner gave a curve which approached the horizontal axis as the cracking progressed. Obviously the greater the area under the curve the greater the degree of protection. Observations were made at intervals usually after 8, 24, 48, 72 and 96 hours. The areas under the curve were then measured with a planimeter and the area divided by the area for a similar stock run at the same time without any anti-flex cracking agent. The resulting I figure is the ratio of the protection as compared to the untreated stock as 100. This value is designated as protection ratio.

Table II Antl-Exposure Cracking Agent Protection Ratio 1, l-Dicyclohexyl-tiphenylurea 1, 3-Dicyclohexylurea 1,1,3,Z-Tetracyclohexylurem. 1,1-1) icyc-lohcxyl-Ii-p-tolylurea 1,1-D icyclohexyl-iS-o-tolylurea 283 1.1-1) icyPlohexy1-3-(2-naphthyi) ur 286 1,1-Dicyclohexyi-3-(2-biphenylyl)urea 281 1,1 ,3-Tricyclohexyl-2-thiourea 175 1,1-Dicyclohexyl-3-phenyl-2-thioure 172 40 ance of standard MgO as 100%.

such as isoprene (e. g. Butyl rubber), specific examples being:

Stock E F Butadieno-acrylonitrile rubber Parts by weight 100 isobutylene-isoprene rubber- -do 100 do-- 60 64 do 4 4 do 1 1 S 1. 5 0. 75 2,2-Dithiobis henzothiazole 1. 5 1. 0 Tetra ethyl thiuram dlsulfide. 1. 5 Lead oxide 5. O 1,l-Dicyelohexyl-3-phenylurea do 2.0

It is obvious from the foregoing that the new compounds of this invention are a class of compounds which substantially increase the life of natural and synthetic rubber goods. The new anti-exposure cracking agents disperse readily and rapidly in rubber stocks, both natural and synthetic.

An important advantage of the anti-exposure cracking agent of this invention is that they do not stain nor discolor the rubber stocks into which they are incorporated. To illustrate this property rubber stocks are compounded com risin Stock "I G l H Pale crepe rubber parts by weight 100 100 Zinc oxided 60 3Q Lithopone. 20 ulfur 2 Diphenylguanidine phthalate... 0.825 2-Benzothiazyl thiol benzoate. 0. 675 Paraflin 0.25 Antiexposure cracking agent 1. 5

The stocks are vulcanized by heating 60 minutes in a press and samples of the cured stocks exposed under an 8-1 sunlamp for various intervals. The light reflected from the surface of the stocks is'measured by means of a photovolt reflectance meter calibrated against reflect- An untreated control (Stock G) is run at the same time. Comparing Stock H to the untreated control shows whether or not the antiexposure cracking agent causes discoloration.

Table III Light Reflectance, Percent Anti-Exposure Cracking Temp.

Agent Stock of Cure, After exposure C. Unexto sunlamp posed 24 Hrs. 10 Days Nongiunfiluulfl."final" G 126 75 75 69 1,1- toys 0 exy -3-p eny urea H 126 71 71 11 Ion ij ....1 h h I G 142 70 68 68 1- icyco exy p enyurea H 142 70 s9 69 G 142 81 79 74 H 142 78 74 None G 126 75 75 69 1,1,3-Tricyclohexyl-2-thiourea H 126 73 72 72 65 None G 126 74 74 73 1, 1 DicycIohexyl-3-phenyl- 2-thiourea H 70 70 70 Smaller amounts of the urea compounds of this invention may be employed than those indicated in the foregoing examplcs. Amounts as small as 0.2% by weight on the rubber (natural and synthetic) of the new compounds of this invention exhibit satisfactory anti-exposure cracking properties. Amounts higher than those specifi- 5 cally shown, as for example up to 5% by weight on the rubber, may be employed depending upon the nature of the urea compound as well as that of the rubber, the other compounding ingredients used and the objectives of the compounder.

By the terms a vulcanized rubber and sulfur-vulcanizable rubber as employed in the appended claims, unless otherwise modified, is meant natural as well as synthetic rubbers which are capable of vulcanization when heated with sulfur and includes latices and reclaims of such materials.

It is to be understood that other desired filling and compounding ingredients may be incorporated in the rubber base along with the anti-exposure cracking agent. For example, there may be incorporated other accelerators, softeners, etc. as well as the customary rubber antioxidants.

While the invention has been described with respect to certain specific embodiments it is not so limited and it is to be understood the variations and modifications thereof obvious to those skilled in the art may be made without departing from the spirit or scope of this invention.

What is claimed is:

1. A sulfur-vulcanizable rubber vulcanizate containing a compound of the structure R\ /H NCX.N RI RII where R and R represent alicyclic hydrocarbon groups and R" represents a hydrocarbon group, and X is selected from the group consisting of sulfur and oxygen.

2. A sulfur-vulcanizable rubber vulcanizate containing a urea compound of the formula RNHCO.-N Cal-I11 2 where R is a hydrocarbon radical.

3. A vulcanized natural rubber composition containing a urea compound of the formula where R is an aromatic hydrocarbon radical.

4. A vulcanized rubber composition containing a urea compound of the formula where R is an aromatic hydrocarbon radical, said rubber being a sulfur-vulcanizable synthetic rubber-like polymer prepared by the polymerization of an aliphatic conjugated diene compound.

5. A vulcanized rubber composition containing a urea compound of the formula where R is an aromatic hydrocarbon radical containing less than 13 carbon atoms, said rubber being a sulfurvulcanizable synthetic rubber-like polymer prepared by the polymerization of an aliphatic conjugated diene compound and an unsaturated mono-olefinic compound which contains the CH2=C group and is copolymerizable therewith.

6. A vulcanized rubber composition containing a urea compound of the formula where R is a hydrocarbon radical, said rubber being a sulfur-vulcanizable copolymer of a butadiene-1,3 hydrocarbon and a vinyl aromatic monomer copolymerizable therewith.

7. A vulcanized rubber composition containing 1,1-dicyclohexyl-3-pheny1urea, said rubber being a butadienestyrene copolymer.

8. A vulcanized rubber composition containing 1,1-dicycloheXyl-3-p-tolylurea, said rubber being a butadienesyrene copolymer.

9. A vulcanized rubber composition containing 1,1-dicyclohexyl-3-(2-naphthyl)urea, said rubber being a but tadiene-styrene copolymer.

10. A vulcanized rubber composition containing 1,1,3- tricyclohexyl-Z-thiourea, said rubber being a butadienestyrene copolymer.

11. A vulcanized rubber composition containing 1,1- dieycloheXyl-3-o-tolylurea, said rubber being a butadienestyrene copolymer.

Sterrett July 6, 1948 Hill et a1 Sept. 8, 1953 

1. A SULFUR-VULCANIZABLE RUBBER VULCANIZATE CONTAINING A COMPOUND OF THE STRUCTURE 